This invention relates to liquid applicators including ball-point pens and paint markers, and caps thereof.
FIG. 6 is an example of a mounting structure of a cap in a conventional liquid applicator. The illustrated cap 80 is formed of an elastically deformable material such as a synthetic resin, and includes a hollow bottom cylindrical portion 81 and a hollow lead cylindrical portion 82 having a smaller diameter than the bottom cylindrical portion 81. The lead cylindrical portion 82 is formed axially continuously with the bottom cylindrical portion 81. An inner circumferential surface 83 of the lead cylindrical portion 82 is formed into a tapered surface whose diameter decreases as it extends toward a lead end of the cap (to the left in FIG. 6). At an inner circumferential surface of a bottom end of the bottom cylindrical portion 81, there are formed a plurality of ribs (four in this illustrated example) 84 which are spaced apart circumferentially and project inward.
A main shaft 90 includes a casing 91 for containing ink therein and a lead portion 92 mounted at a lead end of the casing 91. A nib 93 projects out of a lead end face of the lead portion 92, and a line of projection 94 is formed at an outer circumferential surface of the casing 91. The projection 94 extends entirely over the circumferential surface of the casing 91, and projects outward. In this example, an inside diameter of a circumferential trace of the ribs 84 is larger than an outside diameter of the casing 91 and smaller than an outside diameter of the projection 94. An outside diameter of the lead portion. 92 is smaller than an inside diameter of a bottom end of the lead cylindrical portion 82 and larger than an inside diameter of a lead end of the cylindrical portion 82.
The respective portions are positioned relative to one another as follows. When the cap 80 is mounted on the lead end of the main shaft 90 from the bottom end thereof, the ribs 84 come to contact with the projection 84. Before the ribs 84 moves over the projection 94, the lead end of the lead portion 92 comes to contact with the tapered inner circumferential surface 83 of the lead cylindrical portion 82, thereby holding an interior of the lead cylindrical portion 82 airtight. Thereafter, the ribs 84 move over the projection 94, and the lead portion 92 is completely fitted in the lead cylindrical portion 82.
Both the lead cylindrical portion 82 and the ribs 84 of the cap 80 are fitted to the corresponding portions of the main shaft 90 for the following reasons. The interior of the lead cylindrical portion 82 is held airtight by completely fitting the lead portion 92 into the lead cylindrical portion 82. A user is allowed to confirm the completion of the cap mounting operation by a sound or touch generated when the ribs 84 move over the projection 94.
In the above liquid applicator, an axial force required to move the ribs 84 over the projection 94, i.e. to complete the fitting of the cap to the main shaft, (hereinafter referred to a fitting strength) is a sum of a force to bend the ribs 84 radially outward along the projection 94 and a force required to fit the lead portion 92 compressively into the lead cylindrical portion 82 as much as to displace the cap 80 axially relative to the main shaft 90 when the ribs 84 move over the projection 94. If this fitting strength is exceedingly larger, a large force must be applied so as to mount the cap to the main shaft, with the result that the operability of the liquid applicator is deteriorated. In other words, the fitting strength needs to be adjusted accurately in order to mount the cap 80 properly. In view of this, the cap 80 is not formed with a projection extending entirely over the inner circumference thereof but a plurality of ribs 84 spaced apart circumferentially. This reduces a contact area of the ribs 84 with the projection 94, thereby reducing the fitting strength.
However, in this structure, the user may determine mistakenly that the cap fitting operation has been completed by a touch the user feels when the ribs 84 come to contact with a lead end face of the projection 94 (i.e., immediately before the ribs 84 move over the projection 94), and may stop the cap fitting operation. Since the lead portion 92 is not yet fitted to the inner circumferential surface 83 of the lead cylindrical portion 82 at this time and the ribs 84 are spaced apart without extending continuously circumferentially, the interior of the lead cylindrical portion 82 cannot be held airtight. Thus, the liquid applicator suffers the problems that the nib 93 gets dry and the ink in the main shaft 90 evaporates when being left in this state for a long time.
As a means for avoiding the above problems, it can be considered to form a projection extending continuously circumferentially in place of the ribs 84 and to perform a fine adjustment of the fitting strength by setting a projected amount of this projection. However, in order to adjust the fitting strength finely in this manner, both of the projections must be sized precisely, thereby necessitating an increase in the cost. Further, in this structure, the interior of the lead cylindrical portion 82 is held airtight constantly during a time which lasts until the cap fitting operation is completed after the projection formed in place of the ribs 84 comes to contact with the projection 94. Accordingly, the pressure in the lead cylindrical portion 82 is increased excessively by the axial displacement made in mounting the cap 80 to the main shaft 90. Thus, when the cap 80 is detached from the main shaft 90 next, the pressure around the nib 93 is reduced drastically to as low as an atmospheric pressure. This may cause an excessive ink flow inadvertently.